Originally only optical glass was used as lens material for lens systems and spectacles. With the advances that have been made in the production of optically transparent plastics, such plastics have increasingly been used in lens systems and today predominantly also in spectacles. Plastics as a lens material have requirements similar to optical glasses, that is to say a good optical homogeneity, high spectral transmission, reproducible refractive index values and dispersion, low stress birefringence, low internal scattering, low water absorption, high chemical resistance in the case of terminating elements and high geometrical accuracy in machining and high dimensional accuracy in molding or injection molding. The main requirements for spectacle materials are a low weight, thin geometry as a result of a high refractive index of the material, UV protection, high mechanical load-bearing capacity as protective goggles or sports goggles or spectacles and abrasion resistance as a result of hardness of the substrate or hard protective layers.
A high mechanical load-bearing capacity can be achieved for example by plastics of polycarbonate or polyurethane. Examples of branded products are Trivex® and NXT® polyurethane. The underlying mechanisms of the notch impact strength of the materials Trivex® and NXT® are based inter alia on particles of hard material located therein, whereby a crack that forms has to cover much longer paths through the material, and thereby use up more energy, than in the case of a material without such particles. On the other hand, such particles or microcrystallites in polycarbonates and polyurethanes prevent the formation of functional structures such as gratings or Fresnel structures, since geometrical defects occur at tips, whereby the optical quality suffers. Moreover, depending on size, the microcrystallites are scattered in the visible range, whereby increased stray light occurs, with a disturbing effect in cases of relatively great optical paths.